remoteproc: Check for NULL firmwares in sysfs interface

The remoteproc framework provides a sysfs file 'firmware'
for modifying the firmware image name from userspace. Add
an additional check to ensure NULL firmwares are errored
out right away, rather than getting a delayed error while
requesting a firmware during the start of a remoteproc
later on.

Tested-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>